#region Using Directives and Copyright Notice

// Copyright (c) 2007-2010, Computer Consultancy Pty Ltd
// All rights reserved.
// 
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are met:
//     * Redistributions of source code must retain the above copyright
//       notice, this list of conditions and the following disclaimer.
//     * Redistributions in binary form must reproduce the above copyright
//       notice, this list of conditions and the following disclaimer in the
//       documentation and/or other materials provided with the distribution.
//     * Neither the name of the Computer Consultancy Pty Ltd nor the
//       names of its contributors may be used to endorse or promote products
//       derived from this software without specific prior written permission.
// 
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" 
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// DAMAGE.

using System;
using System.Collections.Generic;
using System.Text;

using Interlace.Utilities;

#endregion

namespace Interlace.Mathematics
{
    public class Pert
    {
        ExtendedRandom _random;

        double _alpha;
        double _beta;

        double _maximum;
        double _minimum;

        public Pert(ExtendedRandom random, double minimum, double mode, double maximum)
        {
            _random = random;

            double mean = (minimum + 4.0 * mode + maximum) / 6.0;
            double stdev = (maximum - minimum) / 6.0;

            _alpha = ((mean - minimum) / (maximum - minimum)) * 
                ((mean - minimum) * (maximum - mean) / (stdev * stdev) - 1);
            _beta = ((maximum - mean) / (mean - minimum)) * _alpha;

            _maximum = maximum;
            _minimum = minimum;
        }

        public double NextVariate()
        {
            if (_minimum == _maximum) return _minimum;

            return _random.BetaVariate(_alpha, _beta) * (_maximum - _minimum) + _minimum;
        }
    }
}
